1. Field of the Invention
The present invention relates to a noise reducing device, and in particular to an active control type noise reducing device that is added to a sound barrier and reduces noise by active control.
2. Description of the Related Art
In Japan, roadways pass through residential areas and alongside hospitals that are supposed to be quiet, and large trucks come and go through these roadways day and night. Motorcycles of motorcycle gangs also travel on these roadways spreading loud explosive noise. The problem of road traffic noise is becoming manifest not only in urban areas but also in rural areas and has become a large social problem.
In recent years, a system called active noise control (ANC) that reduces noise by active control has gathered attention. The noise cancellation principle of ANC is “superposing antiphase sound waves on the original sound waves that are to be canceled”. Namely, as shown in FIG. 1, active noise control reduces the sound pressure level by superposing, on noise A emitted by a noise source, a control sound B emitted from a control sound source.
This noise cancellation principle can be applied to, for example, diffracted noise that is emitted from a noise source, diffracted at the top of a sound bind travels beyond the barrier. As shown in FIG. 2, noise 12 emitted from a noise source S receives the action of diffraction as a wave phenomenon when the noise 12 passes through the vicinity of the top (control point C) of a sound barrier 10. This means that the control point C becomes a new sound source (secondary sound source) with this point at the center. With respect to the control point C, a control sound 16 is emitted from a control sound source (speaker) 14 disposed in the vicinity of the sound barrier 10. In this case, the control sound 16 is produced so that, at the control point C, the noise 12 from the noise source S and the control sound 16 from the control speaker 14 have opposite phases at the same amplitude. Thus, when the noise is observed at an observation point O located in a region at the side of the sound barrier 10 opposite from that of the noise source S, a noise reducing effect equal to or greater than the amount of noise reduced by the sound barrier 10 can be obtained.
Road traffic noise is noise generated by continuously traveling, plural moving sound sources (automobiles). The main noises generated by automobiles traveling on expressways are engine sounds and tire running sounds. These noises are diffracted at the tops of sound barriers along the expressways and are propagated to the expressway environs.
Conventionally, as technology that reduces road traffic noise by ANC, active soft edge sound barriers (ASE sound barriers) have been proposed and utilized (e.g., see the Oct. 7, 2002 issue of Nikkei Business; Acoustical Science and Technology (Acoustic Society of Japan), Vol. 58, No. 12 (2002), pp. 753-760; and Japanese Patent Application Laid-Open Publication (JP-A) Nos. 9-119114, 2001-172925 and 2002-6854). ASE sound barriers form an acoustically soft (complex acoustic reflectivity is −1) boundary at the end (edge) of a sound barrier by ANC, to thereby reduce low-frequency noise of 500 Hz or less mainly diffracted at the top of the sound barrier. The noise cancellation principle here is one where sound is reduced by making the noise impedance Z at the boundary of the top portion of the sound barrier equal to pc, i.e., the same as the acoustic impedance of the air to completely absorb the sound.
However, an ASE sound barrier only exhibits a noise reducing effect in the vicinity of the upper surface of an ASH cell that is the acoustic controller. Thus, there is the problem that, when an ASE sound barrier is used to try to reduce road traffic noise, numerous ASE cells must be disposed along the sound barrier with no space therebetween, and the device becomes large. Also, the noise reducing effect of an ASE sound barrier is at most about 4 dB, which is hardly a sufficient noise reducing effect.